The present invention relates generally to the field of modulation techniques for digital subscriber loop (DSL) technology, and, more particularly, to improving the performance of communication systems using discrete multi-tone (DMT) modulation.
DSL technology refers to a variety of technologies in which highly sophisticated modulation and crosstalk management techniques are used to greatly expand the bandwidth potential over a single pair of copper wires. The Integrated Services Digital Network (ISDN) is generally considered the original DSL application and provides data rates of 192 kbps in a basic rate interface (BRI) configuration and 1.544 Mbps in a primary rate interface (PRI) configuration. Since the introduction of ISDN in the early to mid-980s, DSL technology has evolved to provide faster data rates in various configurations. For example, high bit rate digital subscriber line (HDSL) uses the same modulation technique as ISDN, but on a larger bandwidth thereby allowing HDSL to provide full duplex T1 (1.544 Mbps) or E1 (2.048 Mbps) service across existing twisted pair copper lines without repeaters.
Other manifestations of DSL technology include the following: symmetric digital subscriber line (SDSL), which provides bi-directional communication at data rates ranging from 160 kbps-2.084 Mbps; asymmetric digital subscriber line (ADSL), which provides data rates between 32 kbps-8.192 Mbps downstream to the subscriber and 32 kbps-1.088 Mbps upstream to the network; and very high bit rate digital subscriber line (VDSL), which provides higher data rates the closer the subscriber is to the central office (CO). For example, 13 Mbps is possible within approximately 5000 feet of the CO, 26 Mbps is possible within approximately 3000 feet, and 51 Mbps is possible within approximately 1000 feet.
ADSL technology has been particularly well received by communication service providers, such as phone companies, because it corresponds to the asymmetric nature of most subscribers"" data applications. For example, such services as video on demand, Internet access, home shopping, and multimedia access all generally require high data rates downstream to the subscriber, but relatively low data rates upstream to the network. Moreover, the growth of the Internet in combination with the increased growth of telecommuting have been driving forces in the widespread deployment of ADSL. One particular embodiment of ADSL that has been instrumental in the business success of the technology is rate adaptive digital subscriber line (RADSL). RADSL allows the service provider to adjust the data rate of the DSL communication link according to the need of the application and to account for line length and quality.
The key to the rate adaptation capability of RADSL is the DMT modulation technique. DMT has been selected by the American National Standards Institute (ANSI) as the standard modulation technique for ADSL because it has greater immunity to noise and can provide greater flexibility in data rates than competitive modulation techniques, such as quadrature amplitude modulation (QAM) and carrierless amplitude/phase modulation (CAP). Specifically, DMT uses the frequency spectrum between 26 kHz and 1.1 MHz for data transmission and the spectrum below 4 KHz for plain old telephone service (POTS). The spectrum up to 1.1 MHz is divided into 256 subcarriers or subchannels each with a 4 KHz bandwidth that can be independently modulated from zero to a maximum of 15 bits/Hz. A transmitter, such as a modem, can use these individual channels to dynamically adapt to line conditions to attain the maximum throughput for the communication link (i.e., DSL link). As the throughput of each individual subchannel is optimized, the overall throughput of the communication link is optimized.
To perform this throughput optimization, a modem will first analyze the line conditions (e.g., signal to noise ratio) for the subchannels during initialization. The number of bits to be transmitted per channel are then mapped according to the signal to noise ratios measured for the subchannels. Clearly, more bits will be allocated to those subchannels exhibiting high noise imununity while few or perhaps no bits will be allocated to very noisy channels. The modem will continually monitor the status of the individual subchannels throughout a communication session and will adjust the bit allocation per channel as necessary to maintain optimum throughput.
Thus, to fully realize the benefits of (R)ADSL as provided through DMT, bits must be allocated to the individual DMT subchannels in an efficient and optimal manner. That is, bits must be allocated to the various channels with as fine a granularity as possible to ensure that the maximum throughput capability of each subchannel is fully utilized. Accordingly, what is sought is a system and/or method for optimally allocating bits to the various subchannels in a DMT modulation system.
Certain advantages and novel features of the invention will be set forth in the description that follows and will become apparent to those skilled in the art upon examination of the following or may be learned with the practice of the invention.
To achieve the advantages and novel features, the present invention is generally directed to a communication system in which information is transmitted using DMT modulation. The communication system includes first logic that is used to measure the response of each of the DMT subchannels and second logic that is used to adapt an equalizer filter associated with each of the DMT channels based on the response measurements. The system further includes third logic that measures the noise variance for each of the DMT subchannels. Using the noise variance measurements, fourth logic is used to assign the number of bits for transmission on each DMT subchannel such that total transmit power is minimized for a fixed data rate.
In accordance with one aspect of the invention, the fourth logic uses the following equation for calculating the number of bits, bi, to be allocated on the ith DMT subchannel:             b      i        =                            b          T                N            +                        1          N                ⁢                              ∑                          j              =              1                        N                    ⁢                                    log              2                        ⁢                          σ              j              2                                          -                        log          2                ⁢                  σ          i          2                      ,
where N is the total number of DMT subchannels, bT is the total number of bits to be transmitted, and 941 is the noise variance for said ith DMT subchannel.
The invention can also be viewed as providing a method for allocating bits to individual subchannels in a DMT modulation system. Broadly stated, the method can be summarized by the following steps: (i) measure the channel response for each of the DMT subchannels; (ii) adapt an equalizer filter associated with each of the DMT subchannels based on the measurements obtained in step (i); (iii) measure the noise variance for each of the DMT subchannels; and (iv) using the noise variance measurements obtained in (iii), allocate the number of bits to be transmitted on each of the DMT subchannels such that total transmit power is minimized for a fixed data rate.
The DMT based communication system according to the present invention is advantageous in that bits are allocated to the individual DMT subchannels in a near optimal fashion based on noise measurements that can be obtained during an initialization or training interval. Thus, the invention can be readily incorporated into communication devices, such as modems, without requiring significant architectural or operational changes to the devices.